The present invention relates to a torque sensor and, more specifically, to a torque sensor for an electric power steering system for an automobile.
Electric power steering systems provide assistance to a driver that is turning a steering wheel. An electric power steering system provides assistance by way of an electronically controlled motor in response to detection of the torque applied at the steering wheel. Accordingly, such system needs to detect a steering torque applied at the steering wheel.
FIG. 1 shows one example of a conventional electric power steering system. In general and as shown in FIG. 1, an electric power steering system has a torsion bar 4 fixed coaxially between an input shaft 2 and an output shaft 8 of the steering system. When a driver applies torque at the steering wheel 1, the torsion bar 4 is twisted and a torque sensor 3 sends an electrical signal that is proportional to the applied torque to an electronic control unit (“ECU”) 6. The ECU 6 functions so that electrical current is supplied to the motor 7 based on the signals from the torque sensor 3, so that the motor 7 applies assisting torque to the output shaft 8 by way of a reduction gear mechanism 5. The output shaft 8 steers the tires 12 via tie-rods 11 and a rack and pinion gear mechanism 10, 9.
It is conventional for a potentiometer-based, contact-type of torque sensor to be used for an electric power steering system. It is known for such a potentiometer-based, contact-type of torque sensor to utilize a resistor film on a rotor fixed to the output shaft and sliding contact probes mounted on a rotor fixed to the input shaft. Whenever there is a torque in the system, the torsion bar twists and the rotors containing the resistive film and the contact probes slide with respect to one another, thereby leading to a change in the voltage drop proportional to the twist in the torsion bar. Slip rings are used to transfer input power to the resistor film and to receive the measured torque signal. An example of such a torque sensor is described in U.S. Pat. No. 5,763,793, which issued on Jun. 9, 1998. These sensors can suffer from poor responsiveness because of sliding friction, and they can suffer from a relatively short life due to the limited life of the slip rings and contact probes.
Another type of conventional electric power steering system uses a reluctance-based, non-contact type of torque sensing device. Torque is detected based on a change in reluctance in response to twisting in the torsion bar. An example of such a torque sensing device is explained in U.S. Pat. No. 5,046,372, which issued on Sep. 10, 1991. This type of torque sensing device can be costly and difficult to assemble.